Prior to the present invention, it has been known to modify crystalline polypropylene by reacting it in the presence of a peroxide with maleic anhydride. See, for example, U.S. Pat. No. 3,414,551, which describes (col. 1, lines 35-40) a process involving mechanical working of the polymer in the presence of maleic anhydride and a peroxide, with the addition of heat. The resulting maleated polypropylene is known to "have outstanding utility as an adhesive for adhering crystalline polypropylene to metal" (col. 1, line 24).
Some of the prior art methods do not employ a vigorous mechanical working and a peroxide together when incorporating the maleic anhydride. See, for example, U.S. Pat. No. 3,161,620, which does not use vigorous mechanical working. Generally, the best results in the prior art appear to have been obtained, i.e. the most efficient addition of maleic anhydride to polypropylene is obtained, when the mechanical working is more than mere mixing. Specifically, the conditions should be similar to those employed in manufacturing a "controlled rheology" polypropylene from a high molecular weight crystalline polypropylene by placing the polypropylene in an extruder with a peroxide. See U.S. Pats. Nos. 3,932,368 (Example 2), 3,862,265 and 4,026,967.
Maleic anhydride ("MA") modified polypropylene has been suggested as a laminate (U.S. Pat. No. 4,617,240) and a coating for various metals (see U.S. Pat. No. 4,031,062); however, when used in the interior of washing machines, for example it tends to "disbond" under the influence of detergents. See U.S. Pat. No. 4,599,385 (col. 5, lines 3-29). It is not clear how the "disbonded" material was made. The method of manufacture clearly affects the tenacity of the adhesion, at least to metal surfaces. Graft copolymers of high density polyethylene and ethylenically unsaturated acids such as maleic acid are used as coatings for steel to be processed by drawing and redrawing to make cans, in U.S. Pat. No. 4,452,375.
The reader may also be interested in U.S. Pat. No. 4,461,809, which employs a "peel test" for "gasoline resistance" applied to a maleic anhydride modified propylene-ethylene copolymer adhering to a polymeric base.